Speed, accuracy, and portability have been recurrent and difficult to achieve goals for devices that scan, measure or otherwise collect data about 3D objects for purposes such as reproduction. With the advent of computers, such devices have useful application in many fields, such as digital imaging, computer animation, topography, reconstructive and plastic surgery, dentistry, architecture, industrial design, anthropology, biology, internal medicine, milling and object production, and other fields. These computer-aided systems obtain information about an object and then transform the shape, contour, color, and other information to a useful, digitized form.
The technology currently available for shape digitizing falls into two distinct but related groups: mechanical systems and optical systems. All systems within those two general categories struggle with the basic criteria of speed, accuracy, and portability in measuring and generating information about an object.
A mechanical system acquires data about an object through the use of a probe that has a sensitive tip. The mechanical system scans an object by manually moving its probe tip across the object's surface and taking readings. Generally, the probe connects to a mechanical arm, and the system tracks the probe's position in space using angle measuring devices as the arm moves. The system calculates the position of the probe with coordinates known from the angle measuring devices.
Although mechanical systems scan with generally high accuracy, the rate at which a mechanical system acquires data is relatively slow and can take several hours for scanning and digitizing. A typical mechanical system measures only one point at a time and can digitize only small, solid objects.
As an alternative to mechanical systems, there are several types of optical object shape digitizers which fall into two basic categories: systems based on triangulation and alternative systems. A triangulation system projects beams of light on an object and then determines three-dimensional spatial locations for points where the light reflects from the object. Ordinarily, the reflected light bounces off the object at an angle relative to the light source. The system collects the reflection information from a location relative to the light source and then determines the coordinates of the point or points of reflection by triangulation. A single dot system projects a single beam of light which, when reflected, produces a single dot of reflection. A scan line system sends a plane of light against the object which projects on the object on a line and reflects as a curvilinear-shaped set of points describing one contour line of the object. The location of each point in that curvilinear set of points can be determined by triangulation.
Some single dot optical scanning systems use a linear reflected light position detector to read information about the object. In such systems a laser projects a dot of light upon the object. The linear reflected light position detector occupies a position relative to the laser which allows the determination of a three dimensional location for the point of reflection. A single dot optical scanner with a linear reflected light position detector can digitize only a single point at a time. Thus, a single dot optical scanning system, like the mechanical system described above, is relatively slow in collecting a full set of points to describe an object. Single dot optical scanners are typically used for applications such as industrial engineering. The digitizing speed is usually slow and is limited by the mechanics of the scanning system, i.e., the moving and positioning of the light beam. However, accuracy of these systems can be high. A scanning head can be mounted on a high-precision, but costly, positioning system to take a digitized image of the object's shape with generally good accuracy. However, because of the high cost, slow speed, and lack of flexibility, single dot optical scanners find generally only limited application.
Scan line systems offer one solution to the speed time bottleneck of single point triangulation system. Those systems typically employ a 2D imager, such as a charged coupled device (CCD) camera, for signal detection. The systems project a light plane (i.e., a laser stripe) instead of just one dot and then read the reflection of multiple points depicting the contour of an object at a location that is a distance from the CCD camera and from which the position can be triangulated. Some embodiments of the scan line-type system attach the CCD camera to a rotating arm or a moving platform. During scanning, either the object moves on a known path relative to the camera and laser, or the camera and laser, together, move around the object. In any case, such systems usually depend on this type of fixed rotational movement and typically use a bulky, high-precision mechanical system for positioning. Because of the use of mechanical positioning devices, rescaling flexibility can be very limited, e.g., a scanner designed for objects the size of a basketball may not be useful for scanning apple-sized objects.
Some laser stripe triangulation systems currently available are further limited because the laser stripe stays at a fixed angle relative to the camera and the system makes its calculations based on the cylindrical coordinates of its rotating platform. The mathematical simplicity in such a projection system complicates the hardware portion of these devices as they typically depend on the rotational platform mentioned. Also, the simplified geometry does not generally allow for extremely refined reproduction of topologically nontrivial objects, such as objects with holes in them (e.g., a tea pot with a handle). Full realization of triangulation scanning with a non-restrictive geometry has not been achieved in the available devices.
The laser stripe triangulation systems currently available are also burdened by factors that place upper limits on scanning speed. The laser stripe triangulation systems which use a rotational platform are constrained by the speed at which the platform or arm can rotate the object without moving or shaking it. Some systems take 15 or so seconds to complete a 360.degree. scan. A target object, such as a person or an animal, may have difficulty staying still for such a scan time.
Another speed limitation is that the laser stripe triangulation systems typically can only generate one light stripe per camera image. As laser stripe triangulation systems generate a single laser stripe and project that stripe upon the object, the CCD camera captures an image of the stripe in a frame image--one laser stripe per CCD camera frame. Thus, the collection of laser information in some systems is subject to the speed limitations of the camera.
Additionally, for those optical triangulation systems employing a computer, there is the further problem of processing the incoming data. The CCD camera typically outputs frames of picture information at a rate of 30 or more frames per second. Each frame is composed of a two dimensional frame matrix of pixels and contains, for example, 640.times.480 pixel values of light intensity information. Thus, laser stripe triangulation systems must sort through many megabytes of information. These systems typically require very powerful computers and have sizeable memory requirements. In addition, they take a relatively long time to process the incoming CCD information into a viable set of points concerning the object. The points created can depict the object, but the system that create them are also limited in that they typically do not achieve a sophisticated model of the object.
Apart from optical triangulation systems (single dot or scan line systems), there are alternative optical scanning systems which present a scanning solution different from those employing triangulation techniques. Range meters and multi-camera systems are among those categorized as "alternative" systems. Range meter systems typically use an infrared pulsed laser and mechanical scanning techniques to project a dot laser across an object and then measure the phase delay of the reflected signal. As range meter systems typically incorporate a single dot method of data collection, they generally have the speed limitations that are intrinsic to single-point scanners. Additional accuracy problems occur because depth coordinates are not sufficiently accurate, such that in some systems, when an object is large, ghosts can appear on the scan.
Another type of alternative scanning system is a stereoscopic system which uses several CCD cameras located at known distances from each other. The captured images are processed with a pattern recognition system which maps the various points of an object captured by the cameras, thereby obtaining the shape/contour information. One advanced stereoscopic system uses 16 CCD cameras. Although each camera in such a system has a small exposure time, it takes several minutes to analyze the data for each scan. This can cause the system to delay, sometimes up to six minutes per scan. In this type of system, the device must also project a special grid on an object to obtain reference points for gluing a complete 3D picture. In addition, accuracy is sometimes a problem because stereoscopic scanning relies on light reflecting properties. The systems make assumptions based on Lambertian reflecting properties to determine resolution surface features of the scanned objects. Different surfaces can dictate different results for the same object.
Thus, for devices that scan, measure or otherwise collect data about an object, it would be a substantial advance if a scanner could be created that could rapidly gather highly accurate data concerning a 3D object. It would also be an advance if the device could rapidly process the data in a fashion that did not require a large computing system (and allow for portable embodiments), and after computing, create a descriptive model from the data points collected about the object.